The present invention relates to a cathode arrangement for sputtering the material of a target in a cathode sputtering unit, comprising a frame-shape magnet which is disposed at the target side remote from the surface to be sputtered.
Arrangements of this kind are known, for example, from U.S. Pat. No. 4,166,018. In such prior art, to obtain an as high as possible plasma density in front of the surface to be sputtered, the magnets are so arranged that the greatest possible amount of lines of magnetic force passes across the target plate to emerge from the front side thereof and return therein again. Each line of force thus forms an arc spanning the surface area to be sputtered, and the arrangement is such that the arcs altogether form a sort of a tunnel closed in itself in which the plasma is enclosed during the sputtering operation. The material is sputtered primarily within the area covered by this tunnel. The sputtering efficiency has been substantially increased in this way, however, the disadvantage still remains that the sputtering is irregular, wherefore the target plates must be replaced even before they are completely consumed. With the high prices of many of the target materials, this may be a considerable loss.
To come to a better utilization, it has been provided to arrange a plurality of magnet sets behind the target plate in a way such as to obtain an extension of the magnetic field component parallel to the surface, under a maximum possible portion of the target area; that is, experience has shown that the sputtering rate mainly depends on this parallel component of the magnetic field.
From U.S. Pat. No. 4,180,450, it is known to dispose at the backside of the target plate further magnets, in addition to the first magnet or set of magnets, in such a manner that their direction of magnetization is at an angle of 45.degree. to 90.degree. to the direction of magnetization of the first magnets. This does result in a more regular sputtering of most of the target materials, however, it has been found that even with this, or with other prior art arrangements, it is hardly possible to sputter magnetizable materials, such as nickel or even iron. The reason of this difficulty is that with a magnetizable target, the lines of magnetic force are to a large extent short-circuited by the material and are then missing in the discharge region in front of the target, so that no sufficiently strong discharges can be maintained in that region. The thickness of targets of magnetizable materials known in the prior art is limited to some tenths of a millimeter, at best.
This limitation results from the magnetic energy available in the effective region. That is, to succeed in sputtering ferromagnetic materials, this energy must be high enough to magnetically saturate the target and thus to let a satisfactory amount of lines of force extend thereacross; at a distance of some millimeters in front of the surface to be sputtered, the lines of force passing across should result in a magnetic field component parallel to the surface and having an induction of at least 100 gauss.
It is self-suggesting and has already been proposed, to solve this problem by correspondingly strengthening the magnets. For this purpose, with the present state of the art, only permanent-magnet material of expensive special alloys is available. Component parts of such materials are therefore costly, cannot be worked easily, and, because of their exceedingly strong dynamic effects, are difficult to assemble to a system and to mount.
Prior art sputtering devices assisted by a magnetic field have a further drawback if thicker targets are employed, even if the targets are of non-magnetic materials. In such a case again, due to the greater thickness, the strength of the magnetic field at the front side of the target is unsatisfactory, so that higher striking and operating voltages are needed to obtain sufficiently high plasma densities and thus high sputtering rates. Attempts have been made to solve the problem while maintaining the geometric configuration of the magnet arrangement and field distribution, again by simply adequately increasing the strength of all of the magnets of the assembly, which is possible in principle, however, as mentioned above, entails substantially higher expenses.